The use of a bellows seal within a valve, such as a gate valve, is very common. In such applications, high cycle life is very desirable, but is difficult to achieve because the valve is subjected to high and/or cyclical operating pressures. In order to achieve a high cycle life, the bellows must be thin and flexible. However, in addition to its function as a fluid barrier, the wall of the bellows typically acts as a pressure barrier between the process fluid and the atmosphere requiring that the wall of the bellows be relatively thick and less flexible, thus decreasing cycle life. Various approaches have been developed to pressure balance the bellows in order to overcome the foregoing problem. For example, in U.S. Pat. No. 4,381,648, the annular space between a pair of bellows is balanced with a liquid maintained at a pressure slightly below the working fluid of a Stirling engine by means of an external pressure regulator. U.S. Pat. No. 4,532,766, which is directed to another Stirling engine application, teaches the use of a smaller bellows interposed between the working gas and a pressure compensating fluid to transmit the working gas pressure to the compensating fluid and thus to the seal bellows. In this latter application there is a requirement that the effective area of the seal bellows matches that of the piston which it seals. Such a dependency is unrealistic because under dynamic operating conditions, pressure drops can occur through the serpentine path of the pressure balancing circuit and the fluid make-up circuits. Any differential pressure across the seal bellows will be momentarily amplified because of the shift in the effective area of the bellows. The piston, however, will not change in effective area resulting in a differential area between the seal bellows and the piston. The differential area increases the pressure on the smaller bellows which transmits the pressure to the seal bellows causing a momentary pressure spike on the seal bellows. This ability of a bellows to change effective area in response to a pressure differential is well known in the art.
Another method for pressure balancing a bellows assembly is disclosed in U.S. Pat. No. 4,483,665 which teaches the use of the air that drives the piston, which is in communication with the pumping bellows, to also apply pressure to the outer diameter of the bellows assembly. In this case, the piston is larger than the bellows assembly creating a pressure amplifier. The pressure of the process fluid through the bellows assembly is greater than the air which drives the piston resulting in the bellows assembly being only partially pressure balanced. To achieve complete pressure balancing, the bellows assembly must be in close communication with the pressure-balancing bellows and there can be no tendency to shift the effective area of the bellows assembly as a result of pressure differentials.
Another method for pressure balancing a bellows assembly is disclosed in U.S. Pat. No. 4,889,350 in which a rotary shaft seal and a pair of bellows having different effective areas are connected serially to produce a system with two different effective areas for the Bellows depending upon the direction and magnitude of the differential pressure across the seal. When pressure is greater externally, the larger bellows contracts until the mechanical stop bottoms and renders the larger bellows inactive. When pressure is greater internally, the smaller bellows bottoms and becomes inactive. The objective is to balance the pressure on the seal nose of the active face seal rings to either an internal or an external pressure.
Another bellows pressure balancing method is disclosed in U.S. Pat. No. 2,880,620 in which a pair of bellows of differential area seal a valve stem. In this invention, the lower end of a smaller bellows is attached to the valve stem and the upper end of a larger bellows is attached to the upper end of the smaller bellows and is positioned concentric with, and physically over the outside of, the small bellows " . . . in telescoped assembly . . . " The lower end of the larger bellows is attached to the valve bonnet. The space between the outside of the larger billows and the inside of the bonnet, and the inside of the smaller bellows and the valve stem is filled with an incompressible liquid. The stem passes through the incompressible liquid and through the top of the bonnet via a packing gland. This system effects true pressure balance of the two bellows, but has the distinct disadvantage that the telescopic arrangement of the two bellows creates a deep, convoluted trap for particulates, fine silts and precipitable materials common to many of the chemicals that such a zero leakage seal would contain. For example, petroleum pumped from a well will often contain natural gas (a volatile organic compound--VOC) that must be controlled from entering the atmosphere per the Clean Air Act Amendments (CAAA), and will also commonly contain fine silts and larger particulates that can migrate by force of the swirling currents that occur in the bonnet of a valve, and be deposited in the deep regions between opposing bellows convolutions, causing them to fill and become inoperable. Certain hazardous chemicals, such as ethylene, also requiring zero leakage valves per the CAAA, can form precipitates in the cooler upper regions of the valve bonnet where there is reduced fluid motion as would occur in the deep recesses between two adjacent convoluted bellows. As with the fine silts, precipitates between the convolutions of the two bellows can build to prevent the bellows from moving during valve closure. In both cases, with an accumulation of foreign matter occurring between the two bellows while the valve is open and the system is flowing, subsequent closing of the valve can have catastrophic results. As the outer edges of the inner bellows convolutions descend and attempt to pass the inner edges of the outer bellows which are rising, trapped material can lock the thin, relatively frangible convolutions, resulting in only the short section of lowermost convolutions opening. The excessive stretching of the lowermost convolutions can cause permanent distortion at best, and catastrophic tearing at worst, resulting in premature failure and permanent leakage of the process fluid into the upper chamber, and contamination of the process fluid by the incompressible fluid. The seal ceases to be a zero leakage device, and the process fluid, by virtue of its greater pressure, will be forced through the torn bellows and past the packing, as is common in valves with packing only. This problem can be averted by constructing the seal where the two bellows are not telescoped one outside of the other, but rather are arranged end-to-end.
In view of the foregoing, it has become desirable to develop a bellows seal assembly for a valve or pump wherein the bellows seal is fully pressure balanced permitting virtually any pressure to be applied thereto, whether the pressure is static, cyclical, pulsating, or in surges or spikes, without damage to or reduction in the cycle life of the bellows. It is further desirable that the two pressure balanced bellows be arranged so as not to trap particulates, fine silts and precipitates between the two bellows where they could lock up, but to arrange them so that one is positioned away from the other while retaining a sealable connection therebetween.